A novel three-phase low coefficient of thermal expansion alloy is described in EPO Patent Publication No. 433,072 ('072) published Jun. 19, 1991. The disclosure of the '072 publication provided improved resistance to stress accelerated grain boundary oxygen embrittlement (SAGBO) in combination with a controlled relatively low coefficient of thermal expansion. The alloy of the '072 patent publication also provided excellent notch rupture strength, relatively low density and acceptable impact strength. Specific applications of the '072 alloy include critical structural turbine engine components such as seals, rings, discs, compressor blades and casings. Low coefficient of thermal expansion alloys are often designated for applications that include structural components having close tolerances that must not catastrophically fail.
In the past, turbine engine manufacturers have only required that alloys be notch ductile for use in critical structural applications. Recently, turbine engine manufacturers have been requiring that alloys also be crack growth resistant. INCONEL.RTM. alloy 718 (Registered trademark of alloy produced by Inco Alloys International, Inc.) is an example of a turbine alloy with excellent crack growth resistance. Crack growth resistance allows an alloy to be forgiving of defects, voids and cracks. Furthermore, crack growth resistance facilitates predictability of part life and location of cracks by inspection prior to failure. Unfortunately, low coefficient of thermal expansion superalloys used in combination with alloy 718 have historically suffered from crack growth problems at temperatures of 538.degree. C. (1000.degree. F.). Although the '072 alloy provides excellent notch ductile behavior and excellent resistance to crack initiation, it is highly desirable for an '072 type alloy to have improved crack growth resistance.
INCOLOY.RTM. alloy 909 (Registered trademark of alloy produced by Into Alloys International, Inc.) is being used in structural applications requiring a relatively low coefficient of thermal expansion. A relatively low coefficient of thermal expansion (CTE) is defined for purposes of this specification as being an alloy providing at least a 10% lower CTE than alloy 718 . However, although alloy 909 provides a relatively low coefficient of thermal expansion, alloy 909 does not offer the crack growth resistance of alloy 718 . Furthermore, alloy 909 suffers from extensive oxidation problems at elevated temperatures. Turbine engine components fabricated of alloy 909 and other 900 series alloys must be periodically replaced during scheduled engine maintenance. The replacement of components fabricated out of alloy 909 contributes significantly to the overall cost of maintaining turbine engines. An alloy having relatively low thermal expansion properties in combination with oxidation resistance would facilitate reduction of engine maintenance costs.
It is the object of this invention to provide an alloy with improved crack growth resistance in combination with the properties of SAGBO resistance, controlled coefficient of thermal expansion, notch rupture strength, impact strength, and reduced density.
It is a further object of this invention to provide an alloy that has relatively low thermal expansion in combination with improved oxidation resistance and stability.